There are numerous methods know in the art for producing tubing, i.e. generally tubular extended lengths, from thermoplastics materials. In many cases the presence of surface imperfections, either on the exterior or interior surface, can lead to flaws which can render the tubing unsuitable for certain application. For example, grooves formed in plastic pipe in some cases result in the weakening of the pipe so that it may not be capable of withstanding internal or external forces that may be applied to it. Accordingly, the interior and exterior smoothness of plastic tubing is a factor that consumers generally consider when evaluating such products. In fact consumers will in some cases select one product over the other as a result of minor differences in the interior smoothness of the tubing, even though those differences in smoothness would not lead to any significant difference in the performance properties of the tubing.
Since the exterior of the tubing is generally in contact with suitably shaped metal during the cooling of the extruded tubing, the exterior is generally smooth. The same is not necessarily true for the interior of the tubing. In the formation of plastic tubing in which the extrudate is subjected to vacuum sizing, wherein the interior of the tubing is not exposed to a device which would polish the interior of the tubing as it is cooled, it has been noted that in some cases the interior of the tubing is not as smooth as would be desired.
An object of the present invention is to provide a method for improving the smoothness of a plastic tubing that is formed in a process in which the interior of plastic tubing is not exposed to a device which polishes the interior of the tubing when the pipe is being cooled.
Another object of the present invention is to provide a method of forming tubing having a relatively smooth internal surface without the use of a device which polishes the interior of the tubing as the tubing is cooled.
Still another object of the present invention is to provide plastic tubing having a smooth interior surface.
Other aspects, objects, and advantages of the present invention will be apparent from the following disclosure.